1. Field of the Invention
The present invention is directed to an improved lube oil formulation. More specifically, this invention is directed to a lube oil composition having a reduced phosphorus content which exhibits satisfactory anti-wear and friction reducing properties.
2. Description of Related Art
In current lube oil formulations for internal combustion engines, phosphorus-containing compounds such as zinc dialkyldithiophosphate (ZDDP), are added to the lube oil formulation to provide improved anti-wear properties. However, it has been found that phosphorus from the phosphorus-containing compounds becomes deposited on the catalyst in catalytic converters, thereby decreasing the efficiency of catalytic converters over time and also deactivating the oxygen sensors. As a result, vehicle manufacturers have recommended that engine oil contain reduced phosphorus contents. At the present time, automotive lube oils typically contain a maximum of about 0.10 to about 0.14 weight percent phosphorus. To reduce the rate at which catalytic poisoning occurs by phosphorus, it would be advantageous to reduce the phosphorus content of the lube oils to about 0.08 weight percent, or lower.
The use of metal phosphates and metal carbamates in lube oils is known. For example, U.S. Pat. No. 4,178,258 discloses that molybdenum bis(dialkyldithiocarbamate) can significantly improve the friction and wear properties of lube oil, particularly when the oil contains zinc dialkyldithiophosphate (ZDDP). The patentee also discloses that other additives such as sulfurized calcium phenates may be present in the lube oil. However, the addition of sulfurized calcium phenates to a lube oil is not desirable because it may reduce the fuel economy and anti-rust properties of the lube oil.
As another example, U.S. Pat. No. 3,513,094 discloses the use of metal dithiocarbamates in lube oils in combination with antimony dihydrocarbyldithioate. Patentees also mention the use of sulfurized sperm oil as a lube oil additive.
Yet another example is U.S. Pat. No. 3,923,669 which discloses the use of zinc dialkyldithiophosphate and a neutral barium salt of a petroleum sulfonate diaryldithiocarbamate in a lube oil. However, the use of neutral barium salts of a petroleum sulfonate is not desirable because of barium's toxicity.
In addition, Japanese Patent Publication J54,113,604 discloses the combination of molybdenum sulfide-oxide, ZDDP and dialkyldithiocarbamate in a lubricant for internal combustion engines.
The use of alkyl phenyl sulfides in lube oils is also known (see U.S. Pat. No. 3,929,654 and U.K. Pat. No. 591,283). However, applicants have discovered that not all alkyl phenyl sulfides are effective antiwear agents.
In view of the disadvantages associated with the aforementioned prior art, it would be desirable to have available a simple yet convenient method for decreasing the concentration of phosphorus-containing compounds, such as zinc dialkyldithiophosphate, in lubricating oil so as to decrease the rate at which phosphates are deposited on the catalyst while maintaining superior antiwear capability. It also would be desirable to provide a lube oil having anti-wear and friction reducing properties comparable to presently available lube oils, but having a reduced phosphorus content as well. This applicants have done by discovering a particular combination of compounds, which when added to a lube oil basestock, result in a lube oil having enhanced anti-wear properties.